Imagers typically consist of an array of pixel cells containing photosensors. Each pixel cell produces a signal corresponding to the intensity of light impinging on its photosensor when an image is focused on the array by one or more lenses. These signals may be stored in a memory and displayed on a monitor, manipulated by software, printed to paper, or otherwise used to provide information about the image. The magnitude of the signal produced by each pixel is substantially proportional to the amount of light impinging on a respective photosensor.
Several kinds of imagers are generally known. Complementary metal-oxide-semiconductor (“CMOS”) imagers and charge coupled device (“CCD”) imagers are among the most common. CMOS imagers are discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, all assigned to Micron Technology, Inc.
CMOS or other imagers typically comprise thousands or even millions of picture elements (“pixel”) cells arranged in rows and columns. Each pixel cell typically comprises a photodiode or other photosensitive element configured to convert incident light into an electrical charge. The electrical charges are accumulated in a capacitor or other storage node during an integration period, then readout, converted to a digital value, and combined with other digital pixel values to form an image. The amount of electrical charge accumulated, and therefore the corresponding digital pixel value, depends on the number of photons impacting the photosensitive element during integration. More photons (i.e., brighter light) yield a greater electrical charge and a correspondingly larger digital pixel value. In low light situations, however, there is little difference between the amount of electrical charge accumulatd in a “bright” pixel cell as compared to a “dim” pixel cell. This often yields noisy, poor quality images.
One method for increasing image quality in low-light situations is to increase the integration period, thereby allowing more time for electrical charge to accumulate in “bright” pixels. However, a longer integration period can result in a blurred images due to movement of the imager or the subject during the integration period. Therefore, a method of capturing high-quality images in low-light conditions without increasing integration time is desirable.